GAMG for overset cases

Summary

Solving overset with GAMG does not work since GAMGInterface is not agglomerated so stays zero size. Mapping to zero size is not supported.

Steps to reproduce

Use any overset case. Switch on GAMG (needs algebraicPair as agglomerator). There will be error in GAMGSolverAgglomerateMatrix when trying to restrict the solution to the coarser level (since it has zero 'faces').

Example case

What is the current bug behaviour?

Out of bounds indexing.

Environment information

• OpenFOAM version : feature-overset-coupledPatch branch

Possible fixes

Agglomerate GAMGInterface as e.g. cyclicAMIGAMGInterface.